Sahl al-Tustari was born in the fortress town of
Shushtar (Tustar in Arabic) during the
golden age of the
Abbasid Caliphate, in
Khūzestān Province in what is now southwestern
Iran. Tustari also kept the company of
hadith scholars. It is reported that when he met
Abu Dawud, he said, "O Abu Dawud, I want something from you." Abu Dawud responded, "What is it?" Sahl said, "On the condition that you will fulfill my request if it is possible." Abu Dawud replied in the affirmative. Sahl said, "Show me that tongue with which you narrate the hadiths of the Prophet (peace be upon him) so that I might kiss it." Abu Dawud assented, and Sahl kissed his tongue. This shows the close proximate of early hadith scholars and early Sufis. In these early days when the Sufis were becoming established mostly in
Baghdad (the capital of modern
Iraq), the most notable Sufis of the time elsewhere were: Tustari in southwestern Iran,
Al-Tirmidhi in Central Asia and the
Malamatiyya or "People of Blame".{{cite book An Islamic scholar who commented on and interpreted the Qur'an, Tustari maintained that the Qur'an "contained several levels of meaning", which included the outer or
zahir and the inner or batin. Another key idea that he unravelled was the meaning of the Prophet
Muhammad's saying "I am He and He is I, save that I am I, and He is He", explaining it "as a mystery of union and realization at the center of the Saint's personality, called the
sirr ('the secret'), or the heart, where existence joins Being."{{cite book | title = The Persian presence in the Islamic world ==Works==